Fuel cells are increasingly being used as power sources for electric vehicles and other applications. An exemplary fuel cell has a membrane electrode assembly (MEA) with catalytic electrodes and a proton exchange membrane formed between the electrodes. During operation of the fuel cell, water is generated at the cathode electrode based on electrode chemical reactions between hydrogen and oxygen occurring within the MEA. Efficient operation of a fuel cell depends on the ability to provide effective water management in the system.
Gas diffusion media plays an important role in PEM fuel cells. In general, diffusion media need to wick product water away from the cathode catalyst layer while maintaining reactant gas flow from the gas flow channels through to the catalyst layer. In addition, the proton exchange membrane between the electrodes works best when it is fully hydrated. Accordingly, one of the most important functions of the gas diffusion media is to provide water management during fuel cell operation.
For best water management, it is desirable to provide a gas diffusion medium having a desirable balance of hydrophilic and hydrophobic properties. By providing gas diffusion media with a proper balance of hydrophilic and hydrophobic properties, it is possible to provide different transportation paths for reactant gases and product water and thus prevent flooding in the cell due to excessive accumulation of water in the pores of the gas diffusion media while maintaining proper hydration of the proton exchange membrane, especially from the anode side when water is carried into the cathode side by proton through osmotic drag at high current density. In addition, it will enable utilizing fairly dry inlet reactant gas by maintaining decent amount of liquid water in the gas diffusion media or by in-cell liquid water recycling and thus reduces the capacity requirement for the external humidifier.
It is common in fuel cell technology to add polytetrafluoroethylene (PTFE) to carbon fiber diffusion media. Such addition makes the media more hydrophobic and provides advantages. Various attempts have been made to improve the water management ability of the PTFE coated media, including the coating of an additional microporous layer and/or embedding of wicking materials into the diffusion media.
Accordingly, it would be desirable to provide gas diffusion media having improved balance of hydrophobic and hydrophilic properties that could be exploited to provide efficient water management in fuel cells. It would further be desirable to provide methods of depositing hydrophobic materials such as polytetrafluoroethylene onto such diffusion media such that the hydrophobic and hydrophilic regions can be precisely positioned in order to obtain optimum fuel cell efficiency.